promiscuous: per-interface toggle via netcfg (ip link set promisc)

LinkDevice trait gains is_promiscuous() and set_promiscuous().
EthernetLink stores promiscuous flag (default: false).
Receive path bypasses MAC check when promiscuous enabled —
captures all frames regardless of destination MAC.

netcfg/ifaces/eth0/promiscuous rw:
  cat /scheme/netcfg/ifaces/eth0/promiscuous  →  'off' or 'on'
  echo on  > .../promiscuous  →  enable promiscuous mode
  echo off > .../promiscuous  →  disable

Accepts: on/off/1/0/yes/no/true/false.

Mirrors Linux 'ip link set dev promisc on/off'.
Required for packet capture tools that need to see all traffic.

netstack/src/link/ethernet.rs

@@ -88,6 +88,7 @@ pub struct EthernetLink {
     qdisc_config: QdiscConfig,
     mtu: usize,
     enabled: bool,
+    promiscuous: bool,
     stats: Stats,
     arp_requests: u64,
     arp_replies: u64,
@@ -125,6 +126,7 @@ impl EthernetLink {
             qdisc_config: QdiscConfig::default(),
             mtu: Self::MTU,
             enabled: true,
+            promiscuous: false,
             stats: Stats::default(),
             neighbor_cache: Default::default(),
             arp_requests: 0,
@@ -799,7 +801,7 @@ impl LinkDevice for EthernetLink {
                 (is_for_us, repr.ethertype)
             };
 
-            if !is_for_us {
+            if !is_for_us && !self.promiscuous {
                 continue;
             }
 
@@ -970,5 +972,13 @@ impl LinkDevice for EthernetLink {
     fn remove_neighbor(&mut self, ip: IpAddress) -> bool {
         self.neighbor_cache.remove(&ip).is_some()
     }
+
+    fn is_promiscuous(&self) -> bool {
+        self.promiscuous
+    }
+
+    fn set_promiscuous(&mut self, enabled: bool) {
+        self.promiscuous = enabled;
+    }
 }
 

netstack/src/link/mod.rs

@@ -65,6 +65,12 @@ pub trait LinkDevice {
         false
     }
 
+    fn is_promiscuous(&self) -> bool {
+        false
+    }
+
+    fn set_promiscuous(&mut self, _enabled: bool) {}
+
     fn mtu(&self) -> usize {
         1500
     }